The increased cost for semiconductor manufacturing equipment and factory floor space has driven equipment vendors increasingly to compete on the productivity of their products and thus to have to increase the number of workpieces, such as wafers, that can be processed in any piece of such equipment per hour (throughput). There are three central factors that determine workpiece throughput: the time spent actually processing the workpieces (e.g. removing photoresist, implanting ions, etc.), the number of workpieces that can be simultaneously processed, and the amount of time that elapses between removing processed workpieces from a processing chamber and inserting unprocessed workpieces into the chamber.
In some conventional workpiece processing systems, there may be a significant delay between the time when processed workpieces are removed from a process chamber and the time when the new unprocessed workpieces are provided to the process chamber. For instance, some systems use a single robot arm to remove and insert workpieces. The robot arm must first align with the processed workpiece, remove the processed workpiece from the processing chamber, move to align with a storage area for processed workpieces (which may involve a 180 degree rotation), deposit the processed workpiece, move to align with a storage area containing unprocessed workpieces, retrieve an unprocessed workpiece, move to align with the processing chamber (which may involve a 180 degree rotation) and deposit the unprocessed workpiece in the processing chamber. The cumulative time required for all such steps may be large resulting in a substantial delay between the time when a processed workpiece is removed from the processing chamber and the time when a new unprocessed workpiece is provided to the processing chamber. In addition, each time that a batch containing a given number of workpieces is processed, these workpieces must be removed through a load lock to transit the pressure differential between atmosphere and process pressure and a new batch must be loaded into the processing environment. The time required for removing and loading batches and for pressurizing or evacuating the load lock also decreases throughput.
One system that has been designed to overcome some of the disadvantages of conventional systems is the currently available Aspen.TM. system available from Mattson Technology, Inc. which is used to process semiconductor workpieces. In the current Aspen.TM. system, a workpiece handling robot has two pairs of workpiece support paddles facing in opposite directions as shown in FIG. 1. Two new workpieces are loaded on the paddles on one side of the robot. Then two processed workpieces are removed from the process chamber on the paddles on the opposite side of the robot. The robot rotates once and then deposits the new workpieces in the process chamber and puts the processed workpieces back in the cassette which may hold from 13 to as many as 26 workpieces. Once a cassette of workpieces is processed, the cassette is removed and a new cassette is provided through the load lock mechanism shown in FIG. 2. As shown in FIG. 2, a rotation mechanism is used to exchange cassettes quickly in an outer load lock indicated at 202.
Another system designed to overcome some of the disadvantages of conventional systems is shown in FIGS. 3A and 3B and is described in U.S. Pat. No. 5,486,080. In this system two separate robots 62 and 64 move independently of one another to transport workpieces between an implantation station 25 and load locks 22a and 22b. An intermediate transfer station 50 is used to transfer the workpieces. FIG. 3B is a workpiece path diagram showing the transport steps used to move workpieces in the system. While a first robot transports an unprocessed workpiece from the transfer station 50 to the implantation station 25, a second robot transports a processed workpiece from the implantation station 25 to one of the load locks 22a or 22b. While one load lock is being used for processing, the other load lock can be pressurized, reloaded and evacuated.
While the above systems improve throughput and decrease down time for pressurizing and evacuating load locks, reductions in system size, complexity, and cost while maintaining or improving throughput are still needed. For instance, the system of FIGS. 3A and 3B uses two separate robots and a transfer station all of which take up space. However, it is desirable to decrease the size of workpiece processing systems to the extent possible, because the clean room area used for the system is very expensive to maintain. In addition, separate drive mechanisms which may be used for the two robots would be expected to be more complicated and expensive than a system that employs only one drive mechanism.
In addition to throughput, size, complexity and cost, a fundamental constraint on workpiece handling systems is the necessity to avoid contaminating workpieces. Very small amounts of contaminants, such as dirt or dust can render a workpiece unusable and the size and number tolerance for particulate contaminants continues to decrease as workpiece geometries decrease. Workpiece processing equipment may introduce contaminants in a variety of ways. For example, particles may be shed when two pieces of machinery rub or touch. It is important to minimize the exposure of the workpieces to such contaminants during handling and processing.
It is a particular challenge to design doors that minimize particles generated by friction. Doors open and close to allow workpieces to pass between the ambient (usually a clean room environment) to a sealed (and possibly evacuated) chamber or between two chambers. Opening and closing the doors may involve mechanical mechanisms that create particles or may generate particles when two surfaces are pushed together to close the door. It is desirable to decrease the number of particles generated by such doors to reduce the likelihood of contaminating workpieces. In addition to avoiding contamination, it is desirable in many instances to use a door that does not occupy much space, thereby reducing the overall size of the system and conserving valuable clean room space.
In summary, there is a need for a workpiece handling system with high throughput but that does not entail relatively complicated or expensive mechanisms, or mechanisms that occupy a relatively large amount of space. There is a further need for a workpiece processing system with reduced particle generation and workpiece contamination. Without limiting the foregoing, there is a need for door assemblies for use in such systems which reduce the potential for contamination and occupy a relatively small space. Preferably a workpiece handling and processing system would satisfy all of the foregoing needs.